1. Technical Field
The present invention relates to a diaphragm type pressure sensor.
2. Related Art
Some pressure sensors use a diaphragm. The sensor including a diaphragm (a diaphragm type pressure sensor), which has a pressure-receiving portion bent by a pressure difference applied to both sides thereof, measures a pressure by using a pressure sensitive element detecting the bending.
JP-A-2004-132913 discloses a diaphragm type pressure sensor. The pressure sensor disclosed in JP-A-2004-132913 can measure a relative pressure as well as an absolute pressure. The pressure sensor includes quartz crystal diaphragms at a base side and a lid side. Each diaphragm has a recess. The pressure sensor has an internal space formed by facing the recess of each quartz crystal diaphragm and bonding the diaphragms in a laminated direction. A double-ended tuning fork resonator is disposed in the recess of the quartz crystal diaphragm at the base side in the internal space. That is, the double-ended tuning fork resonator is bonded to the quartz crystal diaphragm at the base side with both ends thereof. The longitudinal direction of resonating arms included in the double-ended tuning fork resonator is along a plane direction of the quartz crystal diaphragm. The quartz crystal diaphragm is bent by an applied pressure, resulting in the double-ended tuning fork resonator being bent. The double-ended tuning fork resonator varies its frequency due to an applied tensile or compressive stress. The pressure sensor measure a pressure based on the frequency change.
A quartz crystal resonator element, such as, a double-ended tuning fork resonator element made of quartz crystal may be used as the pressure sensitive element. In this case, synthetic quartz crystal is used, including crystal structural defects. In wet etching a quartz crystal plate to form the double-ended tuning fork resonator element, the crystal structural defects are selectively etched, producing etch-channels and the like inside the resonator element.
The double-ended tuning fork resonator element including the etch channels shows a higher breaking load when a compressive stress is applied rather than when a tensile stress is applied. That is, when a tensile stress or a compressive stress is applied to the double-ended tuning fork resonator element, the breaking limit becomes larger when a compressive stress is applied rather than when a tensile stress is applied. In general, when a tensile stress or a compressive stress is applied to a material, the breaking limit becomes larger when a compressive stress is applied rather than when a tensile stress is applied.
When such double-ended tuning fork resonator element is used in a pressure sensor, the pressure sensor measures a pressure within a range lower than the breaking limit of the double-ended tuning fork resonator element. That is, the pressure sensor measures a pressure within a range in which the double-ended tuning fork resonator element is not broken. If the breaking limit is small, the range of the pressure sensor being able to measure a pressure is also narrowed. Here, the double-ended tuning fork resonator element varies its oscillation frequency according to the magnitude of a stress applied to. Thus, if the breaking limit is small, a large stress cannot be applied, resulting in a variable oscillation frequency range being narrowed. As a result, the resolving power of a pressure measurement is deteriorated.
In addition, the double-ended tuning fork resonator element receives a large mechanical stress when a tensile stress is applied to it. This stress increases a possibility of the double-ended tuning fork resonator element being broken and may arise a problem in that electrode patterns provided to the double-ended tuning fork resonator element are broken. As a result, the pressure sensor lasts short.